Polymethacrylate resins are old and well known and have been used for many years in the manufacture of contact lenses including those identified as corneal contact lenses. Such lenses have little hydrophilicity, but excellent optical properties.
More recently, hydrophilic contact lenses have been made utilizing as monomers acrylic and/or methacrylic monoesters of polyfunctional alcohols, particularly glycols, which are cross-linked with the various acrylic and/or methacrylic diesters of these polyfunctional alcohols and other monomers. These polymers have the capability of forming a hydrogel with water.
Customarily, contact lenses are shaped in the unhydrated state and subsequently hydrated. A major deficiency in many of the prior art polymers consisting of the aforesaid monomers is their poor working properties in the unhydrated state, such as grinding, lathing, and polishing. These poor properties make themselves evident, for example, in forming the lens by lathing in that the unhydrated polymer is brittle and chips off making it impossible to form a good lens. The chipping can sometimes be reduced by lathing at a very slow speed but this makes the production rate uneconomical. Similarly, during the polishing of the lens which has been formed on the lathe, an excessive polishing time is required and/or polishing rings may be present in the final product. When such problems occur, the "reject rate" of lenses is uneconomically high.
One method of manufacturing which avoids these deficiencies is to form the lens by polymerizing the aforesaid monomer on a rotational mold such as is described in U.S. Pat. No. 3,408,429. However, this molding process limits the scope of the lens design in that the combination of optical curves that can be formed in the contact lens is limited. On the other hand, lathing provides for an infinite combination of shapes.
In addition, the present hydrophilic lenses are generally fragile, can be easily torn or damaged during the handling necessary for cleaning, sterilization, and/or placement in the eye. Furthermore, many of the present hydrophilic lenses cannot withstand the high temperature (100.degree. - 121.degree. C.) heat treatment necessary to effect proper sterilization to ensure against any bacterial growth in the hydrogel. Also, the present hydrophilic lenses also tend to readily accept proteinaceous deposits originating from the eye mucous fluids which degrade the optical quality of the lens and act to abrade the cornea.